Wavetable sound synthesis refers to a process for synthesizing sounds from stored information. A wavetable is a table of data representing samples of sound information. Sounds may be synthesized by reading the samples from the wavetable into a digital to analog converter, thereby producing an analog sound signal representative of the desired sound. All types of sounds may be synthesized using this technique, however, large amounts of data storage are needed to store the individual sound samples. In a process known as looping, sounds are synthesized from a wavetable using fewer data samples. During the looping process, the desired sound signal can be created by repeatedly reading out a portion of a wavetable. This technique forms a continuous data stream that, when converted to analog form, will produce the desired sound signal. As a result of using the looping process, a wide range of sounds can be synthesized from a much smaller wavetable, thereby providing a more efficient way to synthesize sound signals.
Although the looping process improves wavetable efficiency, it can result in side effects detrimental to the quality of the synthesized sound. These side effects occur because the looping process produces artifacts as a result of mismatches, or discontinuities, in amplitude, phase or timbre between the beginning and end of the loop.
Techniques have been used in the past to overcome the problems associated with discontinuities in looped signals For example, one technique is called cross-fading. In a typical looping process, the end of one loop flows directly into the beginning of the next loop. At the meeting point there may exist severe discontinuities causing noticeable artifacts sometimes referred to as "clicks." In cross-fading, the loop end and loop beginning are overlapped. To compensate for the increased signal level at the overlapped portion, the end of the loop is faded out while the beginning of the loop is faded in. Although cross-fading can reduce the effects of discontinuities it does not eliminate them. These reduced, but still noticeable, artifacts are sometimes referred to as "blips."
Another technique used to compensate for the effects of looping is described in "Method and apparatus for producing an electronic representation of a musical sound using extended coerced harmonics" (U.S. Pat. No. 5,466,882). The method disclosed in this patent uses a sinusoidal model to represent the synthesized sound signal. According to the disclosed method, synthesized harmonics are forced into strict harmonic relationship with the fundamental frequency to eliminate phase discontinuities at the loop start and end points. However, forcing the signal to be strictly harmonic is a drastic measure, which can seriously alter the quality of the resulting sound. For example, piano sounds synthesized using this technique can have very poor sound quality.